RF-based identification and location-finding systems for determination of relative or geographic position of objects are generally used for tracking single objects or groups of objects, as well as for tracking individuals. Conventional location-finding systems have been used for position determination in an open outdoor environment. RF-based, Global Positioning System (GPS), and assisted GPSs are typically used. However, conventional location-finding systems suffer from certain inaccuracies when locating the objects in closed (i.e., indoor) environments, as well as outdoors. Although cellular wireless communication systems provide excellent data coverage in urban and most indoor environments, the position accuracy of these systems is limited by self-interference, multipath and non-line-of-sight propagation.
The indoor and outdoor location inaccuracies are due mainly to the physics of RF propagation, in particular, due to losses/attenuation of the RF signals, signal scattering and reflections. The losses/attenuation and scattering issues can be solved (see U.S. Pat. No. 7,561,048) by employing narrow-band ranging signal(s) and operating at low RF frequencies, for example at VHF range or lower.
Although, at VHF and lower frequencies the multi-path phenomena (e.g., RF energy reflections), is less severe than at UHF and higher frequencies, the impact of the multi-path phenomena on location-finding accuracy makes location determination less reliable and precise than required by the industry. Accordingly, there is a need for a method and a system for mitigating the effects of the RF energy reflections (i.e., multi-path phenomena) in RF-based identification and location-finding systems that are employing narrow-band ranging signal(s).
As a rule, conventional RF-based identification and location-finding systems mitigating multipath by employing wide bandwidth ranging signals, e.g. exploiting wide-band signal nature for multi-path mitigation (see S. Salous, “Indoor and Outdoor UHF Measurements with a 90 MHz Bandwidth”, IEEE Colloquium on Propagation Characteristics and Related System Techniques for Beyond Line-of-Sight Radio, 1997, pp. 8/1-8/6). Also, see Chen et al. patent US 2011/0124347 A1 whereby the locate accuracy vs. required PRS bandwidth is shown in Table 1. From this table for 10 meters accuracy 83 MHz of bandwidth is needed. In addition, spatial diversity and/or antenna diversity techniques are used in some cases.
However, the spatial diversity may not be an option in many tracking-location applications because it leads to an increase in required infrastructure. Similarly, the antenna diversity has a limited value, because at lower operating frequencies, for example VHF, the physical size of antenna subsystem becomes too large. The case in point is the U.S. Pat. No. 6,788,199, where a system and method for locating objects, people, pets and personal articles is described.
The proposed system employs an antenna array to mitigate the multi-path. The optionally system operates at UHF in the 902-926 MHz band. It is well known that the linear dimension of the antenna is proportional to the wave length of an operating frequency. Also, the area of an antenna array is proportional to the square and volume to the cube of the linear dimensions ratio because in an antenna array the antennas are usually separated by ¼ or ½ wave length. Thus, at VHF and lower frequencies the size of the antenna array will significantly impact device portability.
On the other hand, because of a very limited frequency spectrum, the narrow bandwidth ranging signal does not lend itself into multi-path mitigation techniques that are currently used by conventional RF-based identification and location-finding systems. The reason is that the ranging signal distortion (i.e., change in the signal) that is induced by the multi-path is too small for reliable detection/processing in presence of noise. Also, because of limited bandwidth the narrow bandwidth receiver cannot differentiate between ranging signal Direct-Line-Of-Sight (DLOS) path and delayed ranging signal paths when these are separated by small delays, since the narrow bandwidth receiver lacks the required time resolution, which is proportional to the receiver's bandwidth (e.g., the narrow bandwidth has an integrating effect on the incoming signals).
Accordingly, there is a need in the art for a multi-path mitigation method and system for object identification and location-finding, which uses narrow bandwidth ranging signal(s) and operates in VHF or lower frequencies as well as UHF band frequencies and beyond.
The track and locate functionality need is primarily found in wireless networks. The multi-path mitigation methods and systems for object identification and location finding, described in U.S. Pat. No. 7,872,583, can be utilized in most of the available wireless networks. However, certain wireless networks have communications standards/systems that require integration of the techniques into the wireless networks to fully benefit from various ranging and positioning signals that are described in U.S. Pat. No. 7,872,583. Typically, these wireless systems can provide excellent data coverage over wide areas and most indoor environments. However, the position accuracy available with of these systems is limited by self-interference, multipath and non-line-of-sight propagation. As an example, the recent 3GPP Release 9 standardized positioning techniques for LTE (Long Term Evolution) standard has the following: 1) A-GNSS (Assisted Global Navigation Satellite System) or A-GPS (Assisted Global Positioning System) as the primary method; and 2) Enhanced Cell-ID (E-CID) and OTDOA (Observed Time Difference of Arrival), including DL-OTDOA (Downlink OTDOA), as fall-back methods. While these methods might satisfy the current mandatory FCC E911 emergency location requirements, the accuracy, reliability and availability of these location methods fall short of the needs of LBS (Location Based Services) or RTLS system users, who require highly accurate locating within buildings, shopping malls, urban corridors, etc. Moreover, the upcoming FCC 911 requirements are more stringent than the existing ones and with exception of A-GNSS (A-GPS) might be beyond the existing techniques/methods locate capabilities. It is well known that the A-GNSS (A-GPS) accuracy is very good in open spaces but is very unreliable in urban/indoor environments.
At the same time other techniques/methods accuracy is severely impacted by the effects of multipath and other radio wave propagation phenomena. Thus, making it impossible to meet the upcoming FCC 911 requirements and the LBS requirements. Listed below are in addition to the DL-OTDOA and E-CID locate techniques/methods. The U-TDOA concept is similar to the OTDOA, but uses Location Measurement Units (LMUs) installed at the cell towers to calculate a phone's position. It is (was) designed for the original 911 requirements. LMU's have only been deployed on 2G GSM networks and would require major hardware upgrades for 3G UMTS networks. U-TDOA has not been standardized for support in 4G LTE or WiMAX. Also, LMUs are not used in LTE deployments. Like other methods the U-TDOA accuracy suffers from the multipath. The LTE standardization groups might forgo the LMUs additional hardware and fashion the U-TDOA after the DL-OTDOA, e.g. UL-OTDOA. Note: DL-OTDOA is standardized in release 9.
Another contender for the upcoming FCC 911 requirements is the RF Fingerprinting method(s). This technology is based on the principle that every location has a unique radio frequency (RF) signature, like a fingerprint's pattern, a location can be identified by a unique set of values including measurements of neighbor cell signal strengths, etc. Fingerprinting does not require additional hardware. However, this technology suffers from the fact that it requires a large database and a long training phase. Also, unlike human fingerprints that are truly unique, because of RF propagation phenomena the RF signature repeats at multiple different locations. Furthermore, the database goes stale, e.g. signature ages quickly as the environment changes, including weather. This makes the task of maintaining the database burdensome. The number of hearable cell towers has significant impact on accuracy—need to obtain readings from multitude (8 or more) towers to get a reasonable accuracy (60 meters, as claimed by Polaris wireless). Thus, in suburban environment the accuracy degrades to 100 meters (see Polaris Wireless Location technology overview, July 29; from Polaris Wireless). Also, there is significant variation (up to 140%) of estimated position with the handset antenna orientation (see Tsung-Han Lin, et al. Microscopic Examination of an RSSI-Signature-Based Indoor Localization System).
While there are several causes of the RF fingerprinting database instability one of the major ones is the multipath. Multipath is highly dynamic and can instantaneously change the RF signature. Specifically, in heavy multipath environment, like indoors—people and elevators movements; furniture, cabinets, equipment places changes will result in a different multipath distribution, e.g. severely impact RF signature. Also, indoors and in similar environments a small change in physical location (in 3 dimensions) causes significant changes in the RF signature. This is result of combination of multipath, which makes RF signature 3 dimensional, and short wavelength that results in significant RF signature changes over distances of ¼ wave. Therefore, in such environments the number of points in the database would have to be exponentially increased.
There exist less accurate location methods, for example RTT, RTT+CID, including ones that are based on received signal strength. However, in latter case RF propagation phenomenon make the signal strength vary 30 dB to 40 dB over the distance of a wavelength which, in wireless networks, can be significantly less than a meter. This severely impacts the accuracy and/or the reliability of methods based on received signal strength. Again, all these methods accuracy is suffering from the multipath.
Accordingly, there is a need in the art for more accurate and reliable tracking and locating capability for wireless networks, which can be achieved through multipath mitigation technology.
Positioning reference signals (PRS) were added in the Release 9 of the LTE 3GPP and are meant to be used by the user equipment (UE) for OTDOA positioning (a type of multilateration). The TS 36.211 Release 9 Technical Specification is titled “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation.”
As noted, PRS can be used by the UE for the Downlink Observed Time Difference of Arrival (DL-OTDOA) positioning. The Release 9 specification also requires neighboring base stations (eNBs) to be synchronized. This removes the last obstacle for OTDOA methods. The PRS also improves UE hearability at the UE of multiple eNBs. It is to be noted that the Release 9 specification did not specify the eNB synchronization accuracy, with some proposals suggesting 100 ns. The UL-TDOA is currently in a study phase and it expected to be standardized in 2011.
The DL-OTDOA method, according to the Release 9 specification, is detailed in U.S. Patent Application Publication No. 2011/0124347 A1 to Chen et al., titled “Method and Apparatus for UE Positioning in LTE Networks.” The Release 9 DL-OTDOA suffers from the multipath phenomena. Some multipath mitigation can be achieved by increased PRS signal bandwidth. However, this consequently results in increased scheduling complexity and longer times between UE positions fixes. In addition, for networks with limited operating bandwidth, such as 10 MHz, the best possible accuracy is about 100 meters, as illustrated in Table 1 of Chen et al. These numbers are the results in a best case scenario. In other cases, especially when the DLOS signal strength is significantly lower (10-20 dB) compared to the reflected signal(s) strength, it results in significantly larger (from two to four times) locate/ranging errors.
Chen et al. describe a variant of the UL-TDOA positioning that is also PRS based, referred to as Up Link Positioning Reference Signal (UL-PRS). Chen et al. proposes improved neighbor cells hearability and/or reduced scheduling complexity, yet Chen et al. do not teach anything that addresses mitigating multipath. As a result, the accuracy by Chen et al. is no better than the accuracy per Release 9 of the DL-OTDOA method accuracy.
According to Chen et al. the DL-OTDOA and the UL-TDOA methods are suitable for outdoor environments. Chen et al. further notes that DL-OTDOA and the UL-TDOA methods do not perform well in indoor environments, such as buildings, campuses, etc. Several reasons are noted by Chen et al. to explain the poor performance of these methods in indoor environments. For example, in Distributed Antenna Systems (DAS) that are commonly employed indoors, whereby each antenna does not have a unique ID.
According to Chen, the end result is that in both: the Release 9 and the cell towers based, like UL-TDOA Chen et al., systems, the UE equipment cannot differentiate between the multiple antennas. This phenomenon prevents the usage of the multilateration method, employed in the Release 9 and Chen UL-OTDOA systems. To solve this problem, Chen et al. adds hardware and new network signals to the existing indoors wireless network systems. Furthermore, in case of an active DAS the best accuracy (error lower bound) is limited to 50 meters. Finally, Chen et al. do not address the impact of multipath on the positioning accuracy in indoor environments, where it is most severe (compared to outdoor) and in many cases results in much larger (2×-4×) positioning errors than claimed.
The modifications taught by Chen et al. for indoor wireless networks antenna systems are not always possible because upgrading the existing systems would require a tremendous effort and high cost. Moreover, in case of an active DAS the best theoretical accuracy is only 50 meters, and in practice this accuracy would be significantly lower because of the RF propagation phenomena, including multipath At the same time, In a DAS system signals that are produced by multiple antennas will appear as reflections, e.g. multipath. Therefore, if all antennas locations are known, it is possible to provide a location fix in DAS environment without the additional hardware and/or new network signals if the signals paths from individual antennas can be resolved. For example, using multilateration and location consistency algorithms. Thus, there is a need in the art for an accurate and reliable multipath resolution for wireless networks.